The bonding of materials is critical in making high performance instruments or devices. The quality of a bonding method is judged, in dependence upon its application, in terms of the precision, the mechanical strength, the optical properties, the thermal properties, the chemical properties, and the process simplicity of the bonding. Three popular bonding methods are optical contacting, epoxy bonding, and high temperature frit bonding.
Optical contacting is a room-temperature process which employs no bonding material, and is thus suitable only for certain precision applications involving surfaces having reasonably good surface figure match. Ideally, if the bonding surfaces are thoroughly cleaned prior to bonding, the resulting interface will have low thermal noise and contain almost nothing susceptible to oxidation, photolysis, and/or pyrolysis. Optical contacting produces bonds which are generally unreliable in strength, however, due to sensitivity to surface chemical contamination (such as, by air-borne molecules) and other environmental factors (such as humidity). In addition, surface figure mismatch almost always exists to some extent. Consequently, strong chemical bonds rarely occur extensively across the interface, and voids are seen sometimes in the interface. The bonds produced by optical contacting do not consistently survive thermal shocks. Typically, optical contacting has a low first-try success rate. In case of failure, de-bonding usually degrades surface quality, and thus lowers success rate in re-bonding.
Epoxy bonding is usually a room-temperature process and has a good success rate for regular room-temperature applications. Because epoxy bonding is typically organic-based, however, the bonding is susceptible to pyrolysis (such as by high intensity lasers) and/or photolysis (such as by ultra-violet light) in high power density applications. The strength of the epoxy bond varies with temperature and chemical environment. Because the resulting wedge and thickness cannot be precisely controlled, epoxy bonding is unsuitable for precision structural work. It creates a relatively thick interface which makes optical-index matching more of a concern in optical applications.
Frit bonding is a high-temperature process which creates a high-temperature rated interface. The interface is mechanically strong and chemically resistant in most applications. Because the frit material is physically thick and thus thermally noisy, it is unsuitable for precision structural work. For example, when optimized for bonding fused silica, frit bonding usually creates good coefficient of thermal expansion (CTE) matching with the substrates at room temperature. The matching usually does not hold to a wider temperature range, however, resulting in strain and stress near the interface. Furthermore, a frit bond is opaque and inapplicable in transmission optics. Due to the high temperature requirement, frit bonding requires high-temperature rated fixturing for alignment, and is thus expensive. It is unsuitable if high temperature side effects, such as changes in the physical/chemical properties of the substrates, are of concern.